1. Field of the Invention
The present invention relates an optical writing control device, an image forming apparatus, and a method for controlling an optical writing device, and particularly to the configuration of a pattern that is drawn for correcting a drawing position of an image.
2. Description of the Related Art
In recent years, digitization of information has been promoted. Therefore, image processing apparatuses such as a printer and a facsimile used for outputting digitized information and a scanner used for digitizing documents are essential. Such an image processing apparatus is often provided with an imaging function, an image forming function, a communication function and the like and thereby configured as a multifunction peripheral that can be used as a printer, a facsimile, a scanner and a copier.
Among these image processing apparatuses, an electrophotographic image forming apparatus is widely used as an image forming apparatus that is used for outputting digitized documents. In an electrophotographic image forming apparatus, a photoconductor is exposed to light to form an electrostatic latent image, the electrostatic latent image is developed using a developer such as toner to form a toner image, and the toner image is transferred onto a sheet to perform sheet-output.
In such an electrophotographic image forming apparatus, adjustment for forming an image within an appropriate area on a sheet is performed by matching timing of exposing a photoconductor to light to draw an electrostatic latent image with timing of conveying the sheet. Further, in a tandem type image forming apparatus which forms a color image using a plurality of photoconductors of different colors, adjustment of exposure timing in each of the photoconductors of the respective colors is performed so that images developed in the photoconductors of the respective colors are accurately superimposed. Hereinbelow, these adjustment processes are collectively referred to as position shift correction.
As a specific method for achieving the position shift correction as described above, there are a mechanical adjustment method that adjusts the arrangement relationship between a light source that exposes a photoconductor to light and the photoconductor and a method using image processing that adjusts an image to be output depending on position shift so that the image is finally formed at a preferred position. In the method using image processing, a correction pattern is drawn and read, and correction is performed on the basis of the difference between timing that is determined according to the design and timing when the pattern is actually read so that an image is formed at a desired position (see Japanese Laid-open Patent Publication No. 2009-069767, for example).
When using the position shift correction method using image processing as described above, in order to correct position shift in a main-scanning direction, a pattern that is inclined relative to a sub-scanning direction is drawn. Japanese Patent Application Publication No. 2009-069767 discloses an oblique line pattern having an inclined line shape and a triangular pattern as examples of such a pattern having inclination. Among these patterns, in order to reduce toner consumption, it is preferred to use the oblique line pattern.
On the other hand, a pattern that is drawn in the position shift correction using image processing as described above is detected by receiving reflected light of a beam emitted onto a surface on which the pattern is drawn. That is, when the position shift correction pattern covers a beam spot, reflected light of the beam changes. The pattern is detected by detecting the change by a light receiving unit.
Therefore, in order to accurately detect the position of a pattern, it is preferred that a change in the amount of received light when the pattern reaches a beam spot be steep. Therefore, it is required to increase the maximum value of the area of a range of covering the beam spot with the pattern as far as possible.
A spot of a beam that is emitted from a light source for detecting a correction pattern has a generally perfect circular shape. However, because the axis of the beam is inclined relative to an irradiation surface, the beam spot projected on the irradiation surface is formed into an elliptical shape corresponding to the inclination of the axis of the beam. Further, there is tolerance in an attached state of a sensor for detecting the correction pattern. Therefore, the angle in the long-axis direction of the ellipse differs between apparatuses. FIG. 19(a) is a diagram illustrating an example of such a beam spot.
As described above, in order to increase the maximum value of the area of a range of covering a beam spot with a pattern as far as possible, the pattern is formed so as to cover a wide area of the beam spot on the irradiation surface. On the other hand, when an oblique line pattern is used as described above and the inclination of the oblique line pattern is deviated from the inclination in the long-axis direction of a beam spot, the range of covering the beam spot with the pattern is made narrow.
FIGS. 19(b) and 19(c) are diagrams each illustrating an example of the range of covering a beam spot with an oblique line pattern. When the inclination angle of an oblique line pattern and the inclination angle in the long-axis direction of a beam spot are close to each other, as illustrated in FIG. 19(b), the oblique line pattern covers a wide area of the beam spot. On the other hand, when the inclination angle of an oblique line pattern and the inclination angle in the long-axis direction of a beam spot largely differ from each other, as illustrated in FIG. 19(c), the range of covering the beam spot with the oblique line pattern is narrow.
Also in the state illustrated in FIG. 19(c), in order to cover a wide area of the beam spot with the oblique line pattern, the width of the oblique line pattern is made wide. However, in this case, toner consumption disadvantageously increases.
In view of the above circumstances, there is a need to achieve reduction in toner consumption associated with drawing of a correction pattern for correcting an image forming position and improvement in the accuracy of pattern detection.